1. Field of the Invention
This invention relates to a process for producing a polymerized toner, more particularly relates to a process for producing a polymerized toner comprising producing a colored polymer particles, which includes a binder resin which is obtained by polymerizing a polymerizable monomer in an aqueous medium and comprises at least a colorant dispersed therein, by polymerization step optionally followed by agglomeration step.
The polymerized toner obtained by the process according to the present invention exhibits excellent characteristic features of charging and imaging. The polymerized toner exhibits excellent characteristic features of toner not only under low temperature and low humidity but also under high temperature and high humidity.
In the present invention, the colored polymer particles are referred as “polymerized toner”, the colored polymer particles with added such as an external additive are referred as “developer”. The developer may be called “electrophotographic toner” or simply “toner”.
2. Description of the Related Art
A developer has been used to visualize electrostatic latent images formed on a photosensitive member in an imaging apparatuses such as laser beam printers and facsimiles and copying machines utilizing electrophotographic or electrostatic recoding principles. The developer is mainly composed of colored particles dispersed with a colorant, a charge control agent and a parting agent in a binder resin.
The colored particles are roughly classified into a pulverized toner obtained by pulverization method and a polymerized toner obtained by polymerization method. The pulverization method fuses and kneads the binder resin, the colorant and the other additive components, and then pulverizes and classifies the kneaded product to give the pulverized toner as colored resin powder. The binder resin is synthesized in advance by polymerization of a polymerizable monomer.
While the polymerization method polymerizes, for example, a polymerizable monomer composition comprising the polymerizable monomer, the colorant and the other additive components in an aqueous medium to give the polymerized toner as colored polymer particles. Suspension polymerization method has been mainly used as such a polymerization method, in addition, other dispersion polymerization and emulsion polymerization methods are also known. The emulsion polymerization method, for example, forms fine particles of resin by emulsion polymerization of the polymerizable monomer in an aqueous medium, and then forms particles having toner particle sizes by chemical agglomeration of a dispersed colorant with an emulsifier and fine particles of resin. The particles are further heated to give spherical colored polymer particles. The emulsion polymerization method requires agglomeration process and is also called as an emulsion polymerizing agglomeration method.
No method completely polymerizes the polymerizable monomer in polymerization process avoiding residue of the unaltered polymerizable monomer in the colored particles and then in the developer. Even a trace amount of residual unaltered polymerizable monomer remaining in the colored particles causes following problems (i) through (v).
(i) The residual polymerizable monomer vaporizes, for example, by heating in fixing process and affects working environment or generates bad odor.
(ii) The residual polymerizable monomer causes blocking of toner during storage.
(iii) The residual polymerizable monomer liable to degenerate fluidity of the toner and affects quality of images.
(iv) The residual polymerizable monomer liable to cause offset.
(v) The residual polymerizable monomer liable to cause filming of the toner on the components of image forming apparatus.
These problems caused by the residual polymerizable monomer are more serious in the polymerized toner than in pulverized toner. The content of the residual polymerizable monomer in the pulverized toner can be easily reduced by heating or drying treatment during the production process of the binder resin.
While the polymerized toner has to remove the residual polymerizable monomer from the colored polymer particles comprising additives such as the colorant, the charge control agent and the parting agent. However, the residual polymerizable monomer is liable to be absorbed in these additives and more difficult to reduce its amount than that in singly used binder resin. In addition, the polymerized toner is liable to agglomerate and restricts the reduction of the residual polymerizable monomer by heat treatment. Recently, requirements for the polymerized toner capable of fixing at lower temperature are increasing to comply with high speed printing or full color printing. However, these low temperature fixing toners are very difficult to reduce the amount of residual polymerizable monomer avoiding agglomeration.
Various countermeasures have been proposed to remove the residual polymerizable monomer from the polymerized toner. For example, blowing of a suspension medium saturated vapor in a suspension comprising the polymerized toner particles (colored polymer particles) after suspension polymerization to distill off the suspension medium was proposed (Japanese Patent Laid-open No. 5-100485). However, the procedure is liable to cause bubbling at the interface between air-suspension solution making stable operation difficult. Furthermore, the procedure often causes problems of declined removal rate of the residual polymerizable monomer due to bubbling and lowered quality of the polymerized toner particles by agglomeration.
A method, in which a silicone de-foaming agent is added to a suspension comprising a polymerized toner particles after polymerization process, and an aqueous medium is distilled off from the suspension while suppressing foaming, was proposed (Japanese Patent No. 2923820). The procedure can remove the residual polymerizable monomer from polymerized toner particles suppressing the foaming at the interface between air and liquid. However, stripping treatment in the presence of a silicone de-foaming agent is liable to cause decline the characteristic features of the obtained polymerized toner. Practically, the polymerized toner obtained by the procedure shows declined charge level and is liable to cause fog and low image density accompanying uneven density.